Reverse osmosis with very hard water: strengths and special cases

6 Ottobre 2022

Reverse osmosis is a drastic water treatment, in fact osmotic membranes have the ability to remove up to 96% of the substances dissolved in water.

It is a treatment that removes most of the dissolved salts, and intervenes effectively where the water is extremely rich in salts and there is a need to reduce them, whether they are harmful or simply in abundance.

Where the presence of nitrates is high, the treatment with reverse osmosis is indicated, as it is indicated in areas where the PFAS rise

Where the water is very hard due to the presence of calcium or magnesium carbonates, treating the water with reverse osmosis makes sense to meet the needs of drinking light.

Be careful not to consider the homogeneous abatement capacity, which is capable of reducing all substances by the same percentage. Osmotic membranes do not always have the ability to stop all the substances dissolved in the water, one of those that “pierce” is the co2 present in the water in the form of carbonic acid

A special case for reasoning

We have recently been contacted by an installer with a particular case.

The osmotic water was weakly sparkling. Having ascertained that there were no co2 addition systems downstream, we asked for the value of the inlet conductivity of the water. 1600 micro Siemens with a hardness over 50 ° F

The mystery was soon explained, the membranes discard the carbonates and not the carbonic acid, where the water is rich in carbonates and is also rich in carbonic acid which is nothing but CO2 in solution in the water. In fact, it is the presence of carbonic acid that prevents limestone from precipitating, and if the carbonic acid evaporates, the limestone precipitates.

In some cases the phenomenon is perceived with a metallic / bitter taste, in extreme cases, as a weak carbonation. When measuring the pH of the permeate, a very low value must be read (often below what is required by the legislation (pH 6.5), we will write later on the subject, this is an indication of the presence of Co2 in the water

How to solve the problem

By adjusting the mixing valve it is possible to mitigate the unpleasant perception, where this adjustment is not possible or sufficient it is necessary to “buffer” the presence of carbonic acid, using a filter with dolomitic calcite. We recommend that you insert an ultrafiltration to protect yourself from the risk of bacterial load.

Very hard water, however, tends to ruin the treatment systems in an anticipated manner.

The electromechanical and mechanical parts of a direct production osmosis plant suffer the wear and tear of time in an anticipated manner if it does not intervene with an effective pre-treatment. The use of filters to inhibit the precipitation of limestone upstream of the system are highly recommended.